A method for feeding, at a constant tension and preset drawing ratio, a thread to a textile machine as a function of the operating step of the latter, forms an object of the present invention. A system for the implementation of the method mentioned above also forms an object of the present invention.
Various types of feeder devices and corresponding operating methods capable of feeding a thread to a textile machine controlling the tension and keeping it constant during the machining process thereof are known to a man skilled in the art. Generally such feeders have a main body which comprises a pulley on which there is wound the thread picked up by a spool, such pulley being actuated by an actuator or electric motor, controlled by an electronic control unit to which there is connected a tension sensor, for example a loading cell. The electronic control unit, as a function of the tension of the measured thread, commands the aforementioned motor accelerating or decelerating the rotational speed of the pulley and thus the speed of the thread, thus being able to control the tension thereof, uniforming it to a preset value.
This family of feeders has the major advantage of not requiring any synchronisation with the textile machine: actually, the device, continuing to measure the tension of the thread is not only capable of controlling the tension thereof during the feeding step but also starting and ending the feeding step in an entirely automatic manner.
Various types of feeder devices (and corresponding operating methods) capable of feeding a thread to a textile machine controlling the speed thereof and keeping it constant during the process are also known to a man skilled in the art. Generally, such feeders are similar to those mentioned further above, but they comprise speed detection means associated to the rotary pulley (still actuated by an electric motor thereof). An electronic control unit is connected to such detection means with the aim of keeping the rotational speed of the pulley (on which the thread is bound) constant. This is obtained as a function of the speed data detected by such means and acting on the aforementioned motor.
This family of feeders has the major disadvantage of requiring an accurate synchronisation with the textile machine: actually, the device compulsorily operates in “electrical axis” with the machine (i.e. as a function of the speed of the latter) given that the speed of the fed thread is as a function of the amount of thread picked up by the machine.
This particularly complicates the step of starting and stopping the feeding and thus the programming thereof; an error when setting the ratio between the aforementioned speed and the pick up speed of the machine may actually lead to the breaking of the thread.
Devices for drawing the thread, for example used on draw-winding machines in the yarn preparation industry are also known. Such devices are generally made up of two pulleys (or rollers) on which the thread is wound; they are positioned in a cascading fashion and they are controlled by an electronic unit which controls the rotation speed thereof with the purpose of maintaining an accurate ratio between the two speeds thereof and thus controlling the drawing ratio of the fed thread. Regarding this, it should be observed that the drawing ratio or simply “draw” is usually defined as R=V1/V2, where V1 is the speed with which the machine picks up the thread and V2 the speed of the thread entering into the second roller.
Thus, a drawing device is actually made up of two feeders at a constant speed and obviously it reveals the same drawbacks related to the required synchronisation with the textile machine mentioned further above.
Processes for producing products which provide for, within the same item, the use of the same type of thread (nylon), but with different counts depending on the area of the product being produced are known. This with the aim of obtaining particular effects (greater compactness, transparency, . . . ). For example, from an application point of view there is known the process of producing pantyhose stockings on circular machines, with small and medium diameter, in which there are generally used two types of thread: the first generally with higher count (for example 28 dtex) for obtaining the body and a second lower count (for example 18 dtex) for obtaining the leg. Thus, such machine should provide for a casing (or creel) capable of housing two reels (one for each part of the product—for example body and leg) for each drop and thus here should be provided for the possibility of selecting, through the appropriate thread guide, the thread to be used as a function of the production process during the actuation (i.e. the part of product being produced).
This need obviously forces the manufacturer to have a warehouse with a wide range of thread counts so as to meet the various productions needs (for example, for producing stockings of different thinkness).
EP1901984 on behalf of the same Applicant describes a method for feeding, at a constant tension, a thread to a processing point of a textile machine (such as for example a machine for manufacturing diapers) far from the reel or equivalent supporting member from which the thread is unwound. The thread is controlled tension-wise in proximity of the reel so as to measure the tension of such thread and control and adjust such parameter to a first predetermined tension value at the beginning of the use by the textile machine immediately upon unwinding the thread from the reel. Second means for measuring the tension are arranged in proximity of the textile machine from which there is obtained the information for setting the operating tension in proximity of the reel; this allows obtaining a desired tension at the machine. This based on a comparison made between the thread tension measured at the entry of the latter and a second predetermined tension value, said comparison being such to maintain said measured tension constantly equal to said second predetermined value over the entire duration of use of the machine.
The aforementioned patent text, regarding this known solution, describes a method and a corresponding device which allow overcoming the problem of uniforming, to a set value, the tension of the thread at the second measurement means, but it is not capable of compensating different subsequent frictions to such tension measurement means, various functions for example due to the use of glue applying device on the machine for manufacturing diapers or thread guides of a circular machine. The different frictions downstream of the machine would thus lead to an increase of the tension with ensuing drawing of the thread (particularly evident with elastic yarns). Thus, in a plurality of threads on which the textile machine operates, the latter could all have the same tension, but be fed with different speeds (the thread is drawn proportionally to the friction tension not compensated by the system due to the fact that it is generated downstream of the second tension sensor) to the machine, thus deteriorating the final quality of the finished product.